X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 141

dimethano-1,2,3,4,5,6,7,8-octahydroanthracene (DMA) are examples where
distinct bands are observed [53].
No new absorption bands are observed in other cases, largely due to
the fact that the strong absorptions of the aromatic donors obstruct the
UV-spectral measurements. For the complex between CBr 4 and TMPD, the
quantitative analyses of the temperature and concentration-dependent ab-
sorptions of the new band at 380 nm afford the extinction coefficient of
ε CT = 3.2

10 3 M -1 cm -1 , as well as the thermodynamic parameters for com-
plex formation:

×

S =- 14 e.u.,and K DA = 0.3 M -1 at
295 K. Such thermodynamic characteristics are similar to those of the dihalo-
gen complexes of as well as those of other acceptors with aromatic donors.
Similar results are also obtained for CBr 4 associates with halide and thio-
cyanide anions [5, 53].

H =- 4.5 kcal M -1 ,

∆ ∆

2.3
Complexes of Halide Anions with Aromatic and Olefinic π-Acceptors

Although non-covalent interactions of anions are one of the most actively ex-
plored areas of supramolecular chemistry [15], the anion sensing and recog-
nition have up to now relied primarily on electrostatic binding or hydrogen
bonding to the receptor [16, 54-61]. However, recent UV-Vis and NMR spec-
tral studies clearly reveal that complex formation takes place in the solutions
between halides and neutral olefinic and aromatic π -acceptors such as those

in Fig. 3 [23, 62].

Fig. 3 π -acceptors and their identification

SolutionsofthealkylammoniumsaltsofCl - ,Br - ,I - in acetonitrile show no
visible absorptions beyond 300 nm. The aromatic -acceptor, tetracyanopy-

razine (TCP) is characterized by strong absorptions in the 220-300 nm range
and a shoulder at 350 nm. However, the electronic spectrum of a mixture of
the bromide salt and TCP reveals a new absorption band at

π

λ CT = 400 nm